How the UP.Link platform works

The architecture of the UP.Link platform leverages the design and infrastructure of the Web. The sections below summarize Web operation and show how UP.Link platform operation compares to it.

---

Web operation

The basic operation of the Web is as follows. Suppose a user chooses the following URL using a conventional Web browser:

http://www.uplanet.com

Figure 2-4 summarizes the resulting interaction. The steps are described below.

FIGURE  2-4.     A typical Web transaction

1. The user chooses a Uniform Resource Locator (URL) in the Web browser.
2. The Web browser parses the URL and sends an HTTP/HTTPS request to the Web server (uplanet.com in this example).
3. The Web server parses the HTTP/HTTPS request, determining what to retrieve. If the URL specifies a static file (as it does in this example), the Web server retrieves the file and prepends an HTTP header to it. If the URL specifies a CGI program, the Web server launches the program.
4. The Web server generates an HTTP/HTTPS response. If the requested URL specifies a static file, the response consists of the requested file (with the HTTP header prepended to it). If the requested URL specifies a CGI program, the response contains the output of the CGI program.
5. The Web browser parses the HTTP/HTTPS response and displays the content to the user.

---

UP.Link platform operation

The UP.Link platform uses the Web model described above, but replaces the Web browser with the combination of the UP.Phone and the UP.Link server.

Suppose a user presses an UP.Phone key that requests an HDML deck with the following URL:

http://updev.uplanet.com/dev/hdml/devhome.hdml

This instructs the UP.Phone to issue a request and begins the interaction summarized in Figure 2-5.

FIGURE  2-5.     A typical UP.Link platform transaction

1. The user presses a phone key that has a URL request assigned to it.
2. The phone sends the request containing the URL and other information (for example, the phone's ID number) to the UP.Link server.
3. The UP.Link unwraps the request and generates a conventional HTTP/HTTPS request for the specified URL. It sends this request to the Web server.
4. The Web server parses the HTTP/HTTPS request, determining what to retrieve. If the URL specifies a static file (as it does in this example), the Web server retrieves the file and prepends an HTTP header to it. If the URL specifies a CGI program (that is, an HDML service), the Web server launches the program.
5. The Web server returns the HDML file (with the prepended HTTP header) or the HDML output from the CGI program.
6. The UP.Link server removes the HTTP header from the HTTP response, and verifies that the HDML is valid. Then it creates a response containing the HDML and sends it to the phone.
7. The phone receives the response. It extracts the HDML from the response and displays the first card in the HDML to the user.

---

"Push" transactions

The Web and the UP.Link platform also support "push" transactions, in which a service asynchronously sends information to the user. "Push" transactions are described in Chapter 5 of the UP.SDK Developer's Guide.